A light-emitting element including an EL layer between a pair of electrodes (also referred to as an organic EL element) has characteristics such as thinness, light weight, high-speed response to input signals, and low power consumption; thus, a display including such a light-emitting element has attracted attention as a next-generation flat panel display.
In a light-emitting element, voltage application between a pair of electrodes causes, in an EL layer, recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance (organic compound) contained in the EL layer into an excited state. Light is emitted when the light-emitting substance returns to the ground state from the excited state. The excited state can be a singlet excited state (S′) or a triplet excited state (T′). Light emission from a singlet excited state is referred to as fluorescence, and light emission from a triplet excited state is referred to as phosphorescence. The statistical generation ratio of S′ to T′ in the light-emitting element is considered to be 1:3. Since the spectrum of light emitted from a light-emitting substance depends on the light-emitting substance, the use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various colors.
To display a full-color image on a display, for example, light-emitting elements of at least three colors of red, green, and blue are necessary. Furthermore, the light-emitting elements are required to consume low power.
Examples of specific methods for displaying a full-color image are as follows: so-called side-by-side patterning in which light-emitting elements that emit light with different colors are separately formed; a white-color filter method in which a white light-emitting element is used in combination with a color filter, and a color conversion method in which a light-emitting element that emits monochromatic light, such as a blue light-emitting element, is used in combination with a color conversion filter. Each of the methods has advantages and disadvantages.